Eleven apple cultivars were differentiated using randomly amplified polymorphic DNA (RAPD) markers obtained by the polymerase chain reaction (PCR). The variability of the technique and of the origin of the DNA extract was analyzed. A set of bands consistent in their presence or absence was chosen to create a differentiating band pattern. A key is proposed by which one can differentiate apple cultivars using commercially available prime.

Isolates of the barley mildew pathogen from the air spora over a large part of Europe and from fields of variety mixtures, were tested for virulence against 12 host resistance alleles . Subsamples were tested for their response to triadimenol fungicide and analyzed for 10 DNA loci using RAPD markers and PCR . There was a large range of haplotypes spread over Europe ; irregularity in the distribution was probably due mainly to non-uniform use of the corresponding host resistances and fungicides . A large range of variation was also detectable within individual fields . Positive gametic disequilibria distorted the distribution of virulence alleles among haplotypes and reduced the number of haplotypes detectable in the sample . Analysis of the spread of the newly selected Val3 allele into different European sub-populations indicated that gene flow throughout the population may be rapid for alleles that have a selective advantage .Fungicide resistance was widespread in areas known for intensive use of fungicides for mildew control . Four classes of fungicide response were detectable and particular virulence haplotypes were found to be characteristic for each class .Variety mixtures used in the former German Democratic Republic (GDR) reduced mildew infection, and thus fungicide use, during the years 1984-1991 despite the limited variation in host resistance among the mixtures . A tendency for complex pathogen races to increase in mixture crops was reversed by the large-scale re-introduction of fungicides for mildew control in 1991 . The mixture strategy appeared to be more successful than using the same resistance alleles in pure monoculture or combining them in a single host genotype .

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Understanding population genetics and evolution within species requires recognition of variation within and between populations and the ability to distinguish between the potential causes of an observed distribution of variation. For this aim several established indices of diversity, and a novel one, were applied to population samples of the barley powdery mildew pathogen, Erysiphe graminis f. sp. hordei. Random spore samples were obtained from the air along transects through regions of interest across large parts of Europe in 1990. Significant geographical differences in diversity of virulence genotypes occurred among regional sub-samples. Diversity was highest in the samples from eastern Germany, Denmark and Austria, whereas the lowest values were found in the samples from Italy, southern France and parts of western Germany. Diversity in the pathogen population was generally related to the degree of diversification of host resistance in time and space, although there was considerable variation in ranking among different measures of diversity. Sensitivity to sample size proved to be the major problem with the use of several established indices of diversity. Working with very large sample sizes we used multiple random subsamples of various smaller sizes to determine how the mean index values changed with changing sample size. The Shannon index proved to be considerably affected by sample size, in contrast to the Simpson index that was therefore used as a global measure of diversity. Limits of confidence were estimated for the Simpson index using the bootstrap method of numerical resampling. The two aspects contributing to global diversity, richness and evenness, were considered separately to allow meaningful interpretation of the Simpson index. Random sub-sampling was used to reduce the influence of sample size differences for these measures. Dissimilarity, a novel measure of diversity for use in plant pathology, indicates the average number of major genes for host resistance against which pathotypes in a sample respond differently. It is thus able to account for the genetic relationship among pathotypes, which is not considered by any other index. The approaches developed in this study help to compare major forces driving evolution of large-scale populations of the barley mildew pathogen.